1. Field of the Invention
This invention relates to a system for starting a loopback test, which an input side controls by forming a loop path in a terminating equipment set between digital data system network (hereafter abbreviated as DDSN) side and a subscriber side.
2. Description of the Related Art
Recently, DDSNs that serve various digital data like pieces of intercorporate information, voices, images, etc. have become used on a wide scale. The maintenance control of digital data systems that connects DDSNs and their remote terminals becomes an important consideration in this situation.
FIG. 1 shows a configuration of the whole digital data system.
In FIG. 1, a DDSN 111 is connected to digital signal zero data port (DS0-DP) 132 of a central office terminal, (COT) 131. A data transmission speed of the DDSN 111 is 64 kbps. The digital signal zero data port 132 is connected to a multiplexer/demultiplexer (MUX/DMUX) 133. The multiplexer/demultiplexer 133 of the central office terminal 131 and a multiplexer/demultiplexer circuit 138 of a remote terminal (RT) 137 are connected via a T1 line (primary rate 1.544 Mbps) 135. The multiplexer/demultiplexer 138 is connected to an office channel unit data port (OCU-DP) 139. The office channel unit data port 139 of the remote terminal 137 and a data service unit (DSU) 157 that accommodates a terminal equipment (TE) 155 are connected via a subscriber's line 153. This subscriber's line 153 serves a subscriber with data transmission speeds of 2.4, 4.8, 9.6 and 56 kbps.
The central office terminal 131 is located a few hundred meters away from the DDSN 111. The length of the T1 line 135 is a few kilometers. The length of the subscriber's line 153 is a few kilometers. The length of a line that connects the terminal equipment 155 to the data service unit 157 is a few tens of meters.
The configuration of the central office terminal 131 and the remote terminal 137 is called a digital loop carrier system (hereafter abbreviated as DLC) which is introduced so that the DDSN 111 can accommodate a remotely located subscriber. The DLC connects the DDSN and the subscriber with a multiplexed loop (T1 line 135) and transmits data of the DDSN 111 or the subscriber's line 153 multiplexed in a maximum of 24 channels, so that subscribers are efficiently accommodated.
FIG. 2 explains a multiplexing method of this digital data system.
In FIG. 2, (a) is a data format of the DDSN 111, (b) is a data format of the T1 line 135. The multiplexers/demultiplexers 133 and 138 multiplex data ((a)) having a data transmission speed of 64 kbps over 24 channels ((b)) and demultiplex data ((b)) having a data transmission speed of 1.544 Mbps into data ((a)) having a data transmission speed of 64 kbps.
This digital data system offers a loopback test to detect a loop severance for a system maintenance control.
FIG. 3 shows the configuration of a circuit part that executes a loopback test loaded on the digital signal zero data port (DS0-DP) 132.
In FIG. 3, a lineside loopback detector (hereafter abbreviated as LLD) 311 detects a loopback control code received from the DDSN side and outputs a loopback starting signal if the code matches a predetermined loopback control code. A lineside loopback point (hereafter abbreviated as LLP) 313 forms a loopback path by responding to this loopback starting signal. A dropside loopback detector (hereafter abbreviated as DLD) 315 and a dropside loopback point (hereafter abbreviated as DLP) 317 work similarly against a loopback control code received from a multiplexer/demultiplexer side (a subscriber side).
In a loopback test from the DDSN side, a path is formed with DLP 317, LLD 311, LLP 313, DLD 315 and DLP 317. Also, in a loopback test from the demultiplexer side (a subscriber side), a path is formed with LLP 313, DLD 315, DLP 317, LLD 311 and LLP 313.
As described above, a loopback test begins by starting a loopback at a designated point and sending and receiving a predetermined signal.
Incidentally, the loopback test is controlled by a starting system (latching loopback), where the loopback point that has once formed a loopback path maintains the current condition until the loopback point receives an order to clear the loopback.
FIG. 4 explains a tandem connection for offering service to a distant place.
In FIG. 4, DDSN 410 is connected to DLC 420 equipped with a central office terminal (COT) 131-1 and a remote terminal (RT) 137-1. DLC 420 is connected to DLC 430 equipped with a central office terminal 131-2 and a remote terminal 137-2 via an trunk line (64 kbps) 440. DLC 430 is connected to a data service unit 457 via a subscriber's loop 453.
In essence, the tandem connection is a method for sequentially connecting more than one DLC via a trunk line so that a subscriber located far away from DDSN 410 can be accommodated.
In this tandem connection method, a loopback point is established in a central office terminal and a remote terminal of each DLC and a loopback test is executed in a manner similar to the case described above.
FIG. 5 explains the loopback test started by a DDSN side shown in FIG. 4.
In a loopback test from DDSN 410 to digital signal zero data port 132B, a loopback path is formed according to a loopback control code at digital signal zero data port 132B (L1).
When a return path is formed due to a closed-circuit failure of a channel between the central office terminal 131-1 and the remote terminal 137-1, a loopback control code sent from the central office terminal 131-1 is returned without being received by the remote terminal 137-1 (L2) and is received by the central office terminal 131-1.
The central office terminal 131-1 makes a closed loop within itself, by forming a loopback path from a subscriber side because of a misidentification of this loopback control code as a loopback control code from a subscriber side (L3), and becomes separated from DDSN 410. Thus, the DDSN 410 is made unable to clear a loopback.
As a result, this loopback test misaction has caused a service stoppage that necessitated an actual dispatching of someone to reset the equipment on site to cancel a loopback.
The chances were that such phenomena could occur in large numbers, as the number of loopback points increased in the tandem connection.
Besides, if a loopback action that corresponds to a subscriber side is prohibited in advance so that a misidentification of the loopback starting direction described above is avoided, a loopback test from a subscriber side becomes impossible, which causes a quality deterioration of system maintenance control and makes an installing operation of communication equipment more difficult due to an increase in the number of points which have to be checked at the time of its installation.